There are many tales of men falling in love with mermaids, but very few of them end happily. The marriage always starts out well enough but it isn't long before the mermaid starts to miss her free life among the waves. Mermaids find it very hard to adapt to the dryness of life on the land.
Mermaids are beautiful wives but hopeless as cooks and housekeepers. Even the most lovestruck man must find it irritating to come home to nothing but raw fish for dinner. Because they are vain, a mermaid will neglect the housework and instead spend her day combing her hair and looking in the mirror.
If the man and mermaid have children, the kids will have webbed toes and fingers. They will be excellent swimmers but not very good at most other games.Often when the mermaid abandons the shore she will leave her children behind. She may occasionally return to visit them but, more often than not, she quickly forgets them.
The stories of the Selkies are found exclusively in Faroese, Icelandic, Irish, and Scottish mythology. There are similar tales involving bird women in other countries such as Africa and Russia. In Japan there is the story of the Tennyo which is basically the same. I've read a manga called Ayashi no Ceres which gives an interesting twist to the story although it's rather dark.